The construction industry has typically relied on structural members made of wood for applications such as window and partition framing. One attractive feature of wood in such applications is the ability of wood members to hold screws and nails. This characteristic is advantageous because it permits economical joining of the members to one another and/or other structures, including items of hardware. Wood can also be easily worked in the field using hand tools and can be easily stained or painted to match other wood trim or panelling.
On the other hand, wood is susceptible to damage by fire. This is especially disadvantageous as a result of the recently increased emphasis on fire retarding properties of building materials, particularly commercial structures, as evidenced by modern building codes.
The use of wood framing members for interior glass partitions, also sometimes referred to as "wall screens" or "glazing wall screens," presents particular difficulties during exposure to fire. For example, wood not only degrades relatively rapidly in fire, but it also tends to warp from the heat when exposed to fire, possibly resulting in mechanical failure of the partition wall. Furthermore, radiant heat from the fire passes through the glass and causes combustion of the wood framing members on the unexposed side of the wall.
In addition to its failure to meet fire test standards, other disadvantages are associated with the use of wood as framing material. For example, the supply of clear wood stock is limited and costs have risen as a result. Although techniques have been developed to fabricate longer wood members from a plurality of short pieces of clear wood stock, these techniques introduce additional steps and equipment and hence higher costs.
In view of the above, and particularly due to building codes requiring fire retardant glass partition walls and the like, steel has become the material of choice for many framing applications. Nevertheless, steel members also suffer from shortcomings. For example, steel members expand and distort with great force as temperatures rise in a fire. This causes great difficulty in the use of steel framing as mullions and transoms in glass partition walls because the distortion force exerted by the steel member could shatter the glass during a fire.
In an effort to avoid the difficulties associated with the use of steel and wood framing members in fire rated structures, voids are frequently used between the glass and the steel or wood member. However, this solution causes expensive complications in the design of systems that are also resistant to passage of fire and heat. For example, because of the sacrificial nature of wood and the sever distortion of steel, it is required that complicated and expensive intumescent gasket material be disposed in the voids between the glass and the framing member.
Steel members also possess the disadvantage that they cannot be readily worked at the building site and hence even minor adjustments such as to frame height, for example, cannot be readily accomplished in the field. Furthermore, steel frames are typically prefabricated by welding, making handling difficult and taking up more shipping space than would be required if the frame members could be shipped in a knocked down condition. In addition, steel frames cannot be stained to match the remaining woodwork in a room and accordingly must either be painted or treated with applications which simulate a wood finish.
There is accordingly a need for a framing member which provides adequate fire resistance at a relatively low cost, which can be easily modified at the construction site and which has good screw- and nail-holding properties, as well as an attractive finished appearance. There is also a need to eliminate or substantially reduce the use of intumescent gasket material in fire rated structures. These needs are particularly acute with respect to framing members such as mullions, transoms and beads used to support glass panels and like materials, such as in glass partition walls and in doors having window lites.